The present invention relates to the detection of flame in oil and natural-gas furnaces. In particular, it relates to circuits for processing the signals derived from flame probes or electromagnetic-radiation detectors in order to determine whether or not flame is present.
Various types of sensors are used to detect flames in furnaces. For example, electromagnetic-radiation sensors of various types can be employed to detect the infrared, visible, or ultraviolet radiation emitted from flames and thereby detect flame presence. As another example, flame probes can be used to impress potential differences across the flame area in order to detect currents that are permitted to flow across the potential difference by flame-induced ionization. But the mere presence of ionization current or electromagnetic radiation is not a reliable indication of flame presence, so there remains the question of what to do with the sensor signal in order to be assured of flame presence.
One method of processing the sensor signal is to provide circuits that test the signal for proper amplitude. If enough light or enough current is sensed, then an indication of flame is produced. Though there is some correlation between the output of such a circuit and the presence of flame, the indication that results from this circuit is not entirely satisfactory. Light is not produced only by flames; it can also be produced by glowing furnace walls or ambient light. The flow of current can also be caused by sources other than flame; sometimes there is shorting of the probe to the grounded ignitor horn, causing a large current flow without flame presence. Accordingly, the mere sensing a DC level is not the best means of processing the signal.
Because of the difficulties with the DC signal, some schemes sense alternating currents. Canadian Pat. No. 801,250, for instance, uses a logarithmic detector that indicates flame presence when the ratio of AC-signal amplitude to DC-signal amplitude is sufficiently high. The idea behind this method is ambient ultraviolet light is less likely to flicker than ultraviolet light caused by the presence of a flame. Accordingly, if there is sufficient flicker, then the source of the ultraviolet light must be a flame rather than, say, glowing walls. As a safeguard, this scheme ANDs the logarithmic-detector output with a DC magnitude determination.
Whatever the virtues of this scheme for ultraviolet radiation, it has a drawback when used in a flame-probe apparatus because AC signals can be produced by non-flame sources such as electric ignitors. Canadian Pat. No. 748,015 contains a possible solution to this problem because it proposes detecting AC only in a limited frequency band, a properly selected frequency band should avoid the effect of ignition noise. However, the very limiting of the frequency band could make the requirements of the signal too stringent. Unless the fuel supply is purposely modulated, a flame cannot be counted on always to supply frequency components within the predetermined band.
In light of these limitations in the prior-art devices, it can be appreciated that, while presently known methods have their appropriate applications, a new signal-processing method would be welcome to the art.